In copiers, printers, facsimile machines and the like which are based on electrophotographic technology, organic photoreceptors are widely employed which exhibit excellent advantages such as high sensitivity, small dependence on temperature and humidity, and high speed response to a semiconductor laser beam.
In such electrophotographic photoreceptors, a functionally separated structure is generally employed in that charge generating function and carrier transport function are performed by different materials. According to the structure, the range for selecting materials has been remarkably broadened. Particularly, in organic compounds, it is possible to design many branches of chemical structures and excellent materials for carrier generation and carrier transport have been developed.
As the charge generating materials, a variety of organic dyes and organic pigments have been proposed. For example, there have been known polycyclic quinone compounds represented by dibromanthanthrone, pyrylium compounds and co-crystal complexes of pyrylium compounds with polycarbonates, squarelium compounds, phthalocyanine compounds, azo compounds.
As the carrier transport materials, there have been known compounds having a heterocyclic nucleus containing a nitrogen atom and the condensed ring nucleus represented by oxazole, oxadiazole, thiazole, thiadiazole, imidazole, etc., polyarylalkane series compounds, pyrazoline series compounds, hydrazone series compounds, triarylamine series compounds, styryl series compounds, styryltriphenylamine series compounds, .beta.-phenylstyryltriphenylamine series compounds, butadiene series compounds, haxatriene series compounds, carbazole series compounds and the like. All those carrier transport materials are capable of transporting a positive hole.
When a photoreceptor is prepared by combining a charge generating material with a carrier transport material, conventionally, it has been known that the most durable photoreceptor is prepared when a layered structure is employed in which a charge generating layer comprising the charge generating material is provided on an electrically conductive support and on the resulting layer, a carrier transport layer comprising the carrier transport material is provided.
Because the carrier transport material is capable of transporting a positive hole, in such electrophotographic photoreceptors, operation is carried out while the surface of the photoreceptor is negatively charged.
For charging, a corona discharging method is generally employed which is capable of performing high speed operation and resulting in consistent charge characteristics. The corona discharging generates ozone. In recent years, with an increase in speed of the electrophotographic process, the increase in ozone generation per unit time has been concerned. Therefore, a high durable photoreceptor has been required which adapts to a positive corona discharging process generating a small amount of ozone.
In view of the foregoing, there has been carried out development for organic photoreceptors which enables the layered structure in which an electron transportable carrier transport layer is provided as an upper layer. As the electron transport materials, there are proposed 2,4,7-trinitrofluorenone and those compounds described in Japanese Patent Publication Open to Public Inspection Nos. 1-206349, 2-214866, 5-279582, U.S. Pat. No. 5,468,583, etc.
When those electron transport materials are employed in the conventional electron transportable carrier transport layer, serious trouble is caused in carrier injection characteristics from a charge generating material. Accordingly, light response action as the electrophotographic photoreceptor is deteriorated and the excessive residual electric potential is observed. As a result, it has been impossible to obtain the electric potential contrast necessary for forming images.